Wireless communication networks provide communication services to mobile devices such as cellular telephones, wireless modems, and pagers. A network typically has a set of base stations distributed across a geographical service area. A mobile device accesses the network at any given time through a wireless communication link with one of the base stations. As the device moves through the service area, however, the network must handoff the communication link to other base stations so that the service is not interrupted or degraded. Techniques related to handoff in wireless communication system are disclosed in U.S. Pat. No. 5,920,549 to Bruckert et al., U.S. Pat. No. 5,901,354 to Menich et al., U.S. Pat. No. 5,754,945 to Lin et al., and U.S. Pat. No. 5,657,487 to Doner. These patents are incorporated herein by reference in their entirety for all purposes.
In current wireless networks, the decision to execute a handoff to a nearby base station is typically performed by the network, often using received strength signal indicator (RSSI) information provided by the mobile devices to the base stations. The information provided from a mobile device typically includes values indicating the strengths of signals received at the mobile from several nearby base stations. The communication network then makes a handoff decision according to a hysteresis rule. If the signal power from a nearby base station exceeds by a predetermined hysteresis level that of the base station currently providing the link, then a handoff to the nearby base station is executed. This technique, however, is problematic due to variations in the signal power level that are not correlated to distance from the base station. Because the signal from the mobile device scatters from nearby objects, the signal power experiences variations as the mobile device moves over small spatial scales (i.e., distances on the order of a wavelength). In addition, there are signal power variations as the mobile device moves over larger spatial scales (i.e., distances on the order of building sizes). These variations are caused by shadowing of the signal by local obstructions such as buildings, hills, and the like. In order to mitigate the effect of small scale variations, conventional handoff techniques compute an average of signal samples collected as the mobile device moves over a few wavelengths, and then applies the hysteresis rule to this average signal. This technique, however, only mitigates small scale variations. The average signal still experiences considerable fluctuation due to shadow fading on larger scales. As a result, this local shadow fading often causes inappropriate and unnecessary handoffs that deteriorate the wireless system performance and degrade the quality of service to the mobile device.